The concept of producing plied yarns using the false-twist, self-twist phenomenon is now rather well known in the art. Documents in which the general principles of false-twisting and self-twisting are described include the following:
______________________________________ "Self-Twist Yarn," D. E. Henshaw, Merrow Publishing Co., Ltd., Watford, Herts, England, 1971 RE 27,717 Breen et al 3,225,533 Henshaw 3,306,023 Henshaw et al 3,353,344 Clendening, Jr. 3,434,275 Backer et al 3,443,370 Walls 3,507,108 Yoshimura et al 3,717,988 Walls 3,775,955 Shah 3,940,917 Strachan ______________________________________
For purposes of convenience, some general comments concerning producing plied yarn by these techniques will be described. It is possible to form a plied yarn by false-twisting two or more singles yarn strands, attaching the strands to each other and then permitting the strands to wrap about each other using the release of forces stored by the false-twisting to accomplishing the plying, hence the term "self-twist". The false-twisting itself, in simplified form, involves holding spaced points of a yarn strand and twisting the strand in one direction at a point intermediate the held points, e.g., the center. This produces twists on one side of the center in one direction and on the other side of the center in the opposite direction. The center of the twisted strand constitutes a point of twist reversal and is called a "node". Clearly, forces are stored in the strand in the twisting step. When two strands similarly false-twisted are brought together in side-by-side juxtaposition with their ends held and permitted to act against or with each other by releasing a central node, the stored forces cause the strands to ply, i.e., to wrap around each other spontaneously. The process is enhanced and the product made more stable if the nodes of the two strands are aligned and are joined or locked together before release and plying.
As will be recognized, the torque or twist force exerted by each strand is roughly proportional to the amount of twist therein and that such force decreases as the strands ply. The plying step itself therefore continues until the stored twist forces in each strand decrease to a point at which the remaining twist forces are exactly counterbalanced by the resistance to further twisting in the plied yarn. Thus, if one begins with individual strands and then false twists the strands and plies them, each strand will end up, in the plied yarn, with some degree of false-twist which can be thought of as some remaining stored potential energy, the force exerted thereby being too small to cause further ply twisting against opposing forces in the plied yarn. In a stable plied yarn formed in this fashion, the amount of singles twist always is greater than the amount of ply twist.
Generally speaking, this remaining stored force or energy may not be particularly disadvantageous, depending upon the type of fabric to be produced from the plied yarn. However, when the yarn is to be used to produce certain products such as pile or tufted carpet, the relationship of the remaining twist in the singles yarn becomes highly significant because of the appearance of the product produced therefrom.
Consider, for example, a plied yarn formed from two singles yarns in which the value of singles twist is equal to 1.4 times the value of ply twist (S=1.4 P). In this circumstance it has been found that the fibers in the individual strands are substantially parallel with the axis of the plied yarn. A tufted carpet made from yarn of this description exhibits two significant disadvantages, one of these being the fact that relatively minor variations in the manufacturing process, such as small variations in twist between spindles, causes large changes in the light reflectance characteristics of the carpet tufts, and the carpet can have a highly noticeable streaky appearance which is unacceptable. The second major problem is that with S=1.4 P or more, the individual yarn strands tend to retain their identity as individual strands, even though plied, and tend to split at the tuft tips into separate singles strands which are visible and which give the carpet surface an undesirable "stringy" appearance.
It is also known, as shown in aforementioned U.S. Pat. No. 3,443,370, Walls, to produce a yarn by false-twisting a number of strands greater than two or more assembled threads or yarns, and plying steps with the previously plied yarns to form what can be termed a braid.
However, in forming a braid from previously plied strands, the various forces interact in each plying process in such a way that it is difficult to predict the degree of twist which will exist after all of the steps, and it is therefore a rather complicated process to arrive at an end product which has twist relationships which meet desired specifications.